This invention relates to coal-water suspensions or slurries and to methods for their preparation.
More particularly, this invention relates to a non-settling, soap-stabilized coal slurry having pumping and atomization characteristics similar to those of a heavy fuel oil.
There has been in recent years an increasing interest in coal-water slurries both to produce a pipeline transportable mixture and to burn directly in a boiler as a replacement for oil. Technical problems faced in the design of a suitable slurry include the selection of an appropriate coal particle size range and use of an effective but economical stabilizing agent. The capital and operating costs of a slurry preparation plant must also be as low as possible to insure economic feasibility of slurry production. The slurry must be capable of being transported, stored and pumped without losing its fluid characteristics.
Successful, large scale test burns of coal-water slurries in utility boilers have been reported. A number of different coal-water slurry formulations have been developed including some which assertedly contain coal concentrations as high as 60% or greater. Representative prior art includes the following patents.
Schulz et al disclose in U.S. Pat. No. 4,261,701 a suspension of coal in water stabilized by a material resulting from the reaction of polycyclic, polycarboxylic acids, obtained from the oxidation of coal, with a base. The resulting suspensions were reported to be non-settling for as long as 26 days. Alkali metal salts of organic sulfonic acids, particularly sodium lignin sulfonate, have been used to stabilize lignite-water slurries as is described in U.S. Pat. No. 4,104,035. However, the lignite must be subjected to autoclaving at high temperature and pressure for the stabilizing system to be effective.
Oil-coated coal particles have been stabilized in a water suspension using an imidazoline stabilizing agent as is taught in U.S. Pat. No. 3,210,168. Clancy et al in U.S. Pat. No. 3,762,887 obtained a homogeneous, stable coal-water slurry without use of stabilizing agents through selection and control of the coal particle size distribution.
A last patent, U.S. Pat. No. 2,668,757, although directed to carbon-hydrocarbon liquid slurries, is worthy of note. Carbon particles such as petroleum coke of relatively small particle size were dispersed in hydrocarbon liquids including kerosene and industrial white oils to form a non-settling suspension stabilized by an alkali metal soap of a higher fatty acid. The patent teaches that, to be effective, the soap should be formed in situ. That is the soap is formed by reaction of a fatty acid dissolved in the liquid hydrocarbon to react with an alkali metal or with certain alkali metal derivatives such as the alcoholates or amides.